1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems having a plurality of elevator cars under group supervisory control.
2. Description of the Prior Art
It is common in prior art elevator systems which include a plurality of elevator cars under the supervision of group supervisory control, to improve elevator service by including one or more parking features. For example, in a zone oriented system, a non-busy zone car may be parked at a predetermined floor of its zone with its doors closed and the hall lanterns off. In another type of system, a non-busy car may be parked at a floor located approximately at the midpoint of the building, with its doors closed and hall lanterns off. In buildings which regularly vacate a predetermined floor at a predetermined time of the day, a clock may be used to park or "spot" one or more cars at the floor just prior to the start of the service demand from that floor. Demand features may also utilize actual evidence of a demand peak from a floor to automatically dispatch another car to the floor. For example, if two elevator cars leave a floor with a full load within a predetermined period of time, the system may send available cars to the floor, with one parking at the floor with its doors closed and hall lanterns off while another car serves the floor. When the car serving the floor closes its doors in preparation to leave the floor, the parked car will open its doors and turn on its appropriate hall lantern. When a car leaves this floor with less than a predetermined load, it cancels the demand for this floor.
Certain of these methods are statistical in nature. The dispatching logic is never quite sure whether more than one car is needed to satisfy a demand; or, what time of the day a demand will actually occur; or, what duration the demand will have. Still other methods have an inherent time lag, as a demand has to actually occur before the system responds to accommodate it. Further, if a tenant, for security reasons, for example, would like to have a car at a particular floor at a given time of day, the prior art elevator systems do not have an easily implemented arrangement for satisfying this need.